Solid-state organic light emitting diode (OLED) image display devices are of great interest as a superior flat-panel digital display device. These display devices utilize current passing through thin films of organic material to generate light. The color of light emitted and the efficiency of the energy conversion from current to light are determined by the composition of the organic thin-film material. Moreover, the display devices can be made with exclusively transparent materials supporting the integration of other optical devices above and below an OLED display device. U.S. Pat. No. 5,776,623 issued Jul. 7, 1998 to Hung et al., describes such a system.
OLED materials are also responsive to electromagnetic radiation and, when appropriately biased within an electrical circuit, can produce a current dependent on the ambient light. For example, U.S. Pat. No. 5,929,845 issued Jul. 27, 1999 to Wei et al., describes a system which both emits and detects light using an organic electro-luminescent apparatus. Although the transparent organic emissive materials may emit only a limited frequency range of light, other absorptive materials may be added to the OLED device structure to absorb a very broad spectrum and respond photo-electrically to the presence of light. Other devices and materials, such as silicon photodiodes are also photo-responsive to light.
The luminous efficiency of the OLED devices often decreases significantly with continuous operation due to the instability of the organic materials. This instability may be as a result of aging, temperature changes, humidity, or other environmental stresses. Luminous efficiency is defined as the ratio of luminous output in units of candelas per square meter to the electrical current passing through the OLED in amperes per square meter. Thus, a degradation of OLED luminous efficiency means that upon usage more and more current must pass through the OLED in order to achieve the same luminance.
Moreover, current control in a display device is problematic, especially as compared to voltage control, requiring more circuitry and thus increasing the complexity of the supporting electronics within the display.
Attempts have been made in the prior art to optimize particular display systems to overcome some of the problems noted above. For example, U.S. Pat. No. 5,216,504 issued Jun. 1, 1993 to Webb et al. describes a digital control device within a video monitor to calibrate or otherwise optimize the display, either with human input or under automated computer control.
Some systems integrate user-controlled control mechanisms to provide more flexible operation or optimal use under varying conditions. For example, brightness and contrast controls are often available on CRT and LCD display devices. These controls can be based on information from the device itself, using a reference pixel within the display. U.S. Pat. No. 5,157,525 issued Oct. 20, 1992 to Eaton et al. describes the use of a reference pixel with separate control to maintain a pre-selected value for contrast or absolute brightness using a feedback arrangement which includes an LCD reference element. The feedback information is determined by measuring the average transmissivity of the LCD material with a photo-detector. U.S. Pat. No. 5,910,792 issued Jun. 8, 1999 to Hansen et al. compares current passing through a resistive layer with that of a current source to provide a feedback compensation for temperature-induced brightness variation. One problem with these approaches is that the feedback does not directly respond to the emissivity of the pixels themselves, or address problems with different types (e.g. colors) of pixels within a display. It is also known to calibrate display devices through the use of external sensors which measure the light output from the display device and construct a calibration table for use by the device. See for example U.S. Pat. No. 5,371,537 issued Dec. 6, 1994 to Bohan et al. This approach has the problem that the sensor device obscures the display during the calibration and is not capable of providing real time operation.
There is a need therefore for an improved detection method for light emission and control for an emissive display.